1. Field of the Invention
The present invention pertains to apparatus and a method useful in the deposition of a coating on a substrate, where the coating is formed from chemically reactive species present in a vapor which is reacted with the substrate surface.
2. Brief Description of the Background Art
Both integrated circuit (IC) device fabrication and micro-electromechanical systems (MEMS) fabrication make use of layers or coatings of material which are deposited on a substrate for various purposes. In some instances, the layers are deposited on a substrate and then are subsequently removed, such as when the layer is used as a patterned masking material and then is subsequently removed after the pattern is transferred to an underlying layer. In other instances, the layers are deposited to perform a function in a device or system and remain as part of the fabricated device. There are numerous methods for depositing a thin film layer or a coating, such as: Sputter deposition, where a plasma is used to sputter atoms from a target material (commonly a metal), and the sputtered atoms deposit on the substrate. Chemical vapor deposition, where activated (e.g. by means of plasma, radiation, or temperature, or a combination thereof) species react either in a vapor phase (with subsequent deposition of the reacted product on the substrate) or react on the substrate surface to produce a reacted product on the substrate. Evaporative deposition, where evaporated material condenses on a substrate to form a layer. And, spin-on, spray-on, or dip-on deposition, typically from a solvent solution of the coating material, where the solvent is subsequently evaporated to leave the coating material on the substrate.
In applications where the wear on the coating is likely to occur due to mechanical contact or fluid flow over the substrate surface on which the layer of coating is present, it is helpful to have the coating chemically bonded directly to the substrate surface via reaction of the species with the surface in order to obtain particular surface properties.
With respect to layers and coatings which are chemically bonded to the substrate surface, areas of particular current interest are those of integrated circuitry, and a combination of integrated circuitry with mechanical systems, which are referred to as micro-electromechanical systems, or MEMS. Due to the nanometer size scale of some of the electrical devices formed, and the use of MEMS in applications such as the biological sciences, where the type and properties of the coating on the substrate surface is used to provide a particular functionality to the surface, a need has grown for improved methods of controlling the formation of the coating or layer on the substrate surface. Historically, these types of coatings were deposited in the liquid phase, resulting in limited film property control and loss of device yield due to capillary forces. More recently, vapor-phase deposition has been used as a way to replace liquid processing and to improve coating properties.
For purposes of illustrating a few of the many potential applications for vapor phase coatings, which must either be deposited to have particular critical properties and/or to have particular permanent structural orientation relative to the underlying substrate, applicants would like to mention the following publications and patents which relate to methods of coating formation. Applicants would like to make it clear that some of this Background Art is not prior art to the present invention because it has been published at such a time that it is subsequent to the date of invention for applicants' invention. It is mentioned here because it is of interest to the general subject matter.
Product applications employing coatings deposited on a substrate surface from a vapor include the following, as examples and not by way of limitation. U.S. Pat. No. 5,576,247 to Yano et al., issued Nov. 19, 1996, entitled: “Thin layer forming method where hydrophobic molecular layers preventing a BPSG layer from absorbing moisture”. U.S. Pat. No. 5,602,671 of Hornbeck, issued Feb. 11, 1997, which describes low surface energy passivation layers for use in micromechanical devices. In particular, an oriented monolayer is used to limit the Van der Waals forces between two elements, reducing the attraction between the surfaces of the elements. An article by Steven A. Henck in Tribology Letters 3 (1997) 239-247, entitled “Lubrication of digital micromirror devices”, describes nearly fifty lubricants which were investigated for use in a digital micromirror device. The lubricants included self-assembled monolayers (SAMs), fluids, and solid lubricants. The lubricants were used to reduce the adhesion between contacting surfaces within a microelectromechanical system (MEMS) device. In an article entitled “Vapor phase deposition of uniform and ultrathin silanes”, by Yuchun Wang et al., SPIE Vol. 3258-0277-786X(98) 20-28, the authors describe uniform, conformal, and ultrathin coatings needed on the surface of biomedical microdevices such as microfabricated silicon filters, in order to regulate hydrophilicity and minimize unspecific protein adsorption. Jian Wang et al., in an article published in Thin Solid Films 327-329 (1998) 591-594, entitled: “Gold nanoparticulate film bound to silicon surface with self-assembled monolayers, discuss a method for attaching gold nanoparticles to silicon surfaces with a SAM used for surface preparation”.
Patrick W. Hoffman et al., in an article published by the American Chemical Society, Langmuir 1997, 13, 1877-1880, describe the molecular orientation in monomolecular thin organic films and surface coverage on Ge/Si oxide. A gas phase reactor was said to have been used to provide precise control of surface hydration and reaction temperatures during the deposition of monofunctional perfluorated alkylsilanes. Although some process conditions are provided, there is no description of the apparatus which was used to apply the thin films. T. M. Mayer et al. describe a “Chemical vapor deposition of fluoroalkylsilane monolayer films for adhesion control in microelectromechanical systems” in J. Vac. Sci. Technol. B 18(5), September/October 2000. This article mentions the use of a remotely generated microwave plasma for cleaning a silicon oxide substrate surface prior to film deposition, where the plasma source gas is either water vapor or oxygen. U.S. Pat. No. 6,203,505 to Jalisi et al., issued Mar. 20, 2001 describes guide wires having a vapor deposited primer coating. The guide wires are an intraluminal device having an adhesive primer coat formed of a carbonaceous material and a lubricious top coat of a hydrophilic polymeric material. One preferred coating method for applying a carbon-based primer coating is chemical vapor deposition. The coating is a plasma polymerized coating, so that the resulting polymer is an amorphous structure having groups in the structure other than the monomer groups of the source materials. For example, plasma polymerized polyethylene may include a variety of functional groups, such a vinyl, in addition to the methylene groups. In their article entitled: “Amino-terminated self-assembled monolayer on a SIO2 surface formed by chemical vapor deposition”, J. Vac. Sci. Technol. A 19(4), July/August 2001, Atsushi Hozumi et al. describe the formation of self-assembled monolayers (SAMs) on n-type Si (100) wafers which were photochemically cleaned by a UV/ozone treatment, whereby a thin SiO2 layer was formed on the silicon surface. The SAM coating was applied by placing a cleaned wafer together with a silane liquid precursor diluted with absolute toluene into a container having a dry nitrogen ambient atmosphere. The container was sealed with a cap and heated in an oven maintained at 373° K.
International Patent Application No. PCT/US01/26691, published on Apr. 11, 2002, describes substrates having a hydrophobic surface coating comprised of the reaction products of a chlorosilyl group compound and an alkylsilane. In a preferred embodiment, a hydrophobic coating is formed by the simultaneous aqueous vapor phase deposition of a chloroalkylsilane and a chlorosilyl group containing compound to form an anchor layer, which may then be capped with a hydrophobic coating. The reactants are said to be vapor-deposited simultaneously in a closed humidity-controlled chamber. Dry air, humid air, or dry air saturated with coating precursor vapor was introduced at one end of the chamber and exhausted at the other. The reaction precursors are said to be introduced into the reaction chamber by flowing dry air over the precursor liquid and into the chamber. U.S. Pat. No. 6,383,642 to Le Bellac et al., issued May 7, 2002 described formation of a hydrophobic/oleophobic coating on a substrate such as a glass or plastic material. The coating precursor is introduced into a chamber which employs a pulsed plasma, with the frequency of the plasma generation source ranging from 10 kHz to 10 GHz at a power from 100 to 2000 W, where the substrate surface area to be coated is 0.4 M2. The precursors are introduced into the chamber at various flow rates to establish and maintain a pressure in the chamber ranging from 0.1 to 70 Pa.
W. Robert Ashurst et al., discuss a method of applying anti-stiction coatings for MEMS from a vapor phase in an article published by Elsevier Science B. V., in Sensors and Actuators A 104 (2003) 213-221. In particular, silicon (100) samples cut from a P-doped, n-type test wafer are rinsed in acetone and then cleaned by exposure to UV light and ozone for 15 minutes. The samples are treated with concentrated HF for 10 minutes and then cleaned again as described above before introduction to a vapor deposition chamber. In the vapor deposition chamber, the silicon substrates are additionally cleaned of any organic contamination using an oxygen plasma which is generated in the coating chamber, but at a sufficient distance away from the samples that the samples can be contacted by plasma species without being inside the plasma discharge area. After O2 plasma exposure was begun, water gas was dosed into the chamber and eventually displaced the oxygen. The water was added to form —OH surface terminations oil the substrate surface. The coating was applied by first admitting water vapor to the chamber until the pressure in the chamber exceeded 5 Torr. Subsequently, the chamber was evacuated down to the desired water vapor pressure between 1 and 1.3 Torr. Next a dimethyldichlorosilane (DDMS) precursor was introduced into the process chamber until the total pressure was in the range of 2.5-3 Torr. The reaction was carried out for 10-15 minutes, after which time the chamber was pumped out and vented with nitrogen. It was concluded that increasing substrate temperature during coating over a range of 20° C. to 50° C., all other variables being equal, results in films that have decreasing water contact angle. The main result of the temperature experiments is said to be that there is no need to heat the sample. In a second article entitled: “Vapor Deposition of Amino-Functionalized Self-Assembled Monolayers on Mems”, Reliability, Testing, and Characterization of MEMS MOEMS II”, Rajeshuni Ramesham, Danelle M. Tanner, Editors, Proceedings of SPIE Vol. 4980 (2003), authors Matthew G. Hankins et al. describe microengine test devices coated with films made from amino-functionalized silanes. The coatings were applied in a vapor-deposited self-assembled monolayer system developed at Sandia National Laboratories. The process variables used to deposit the coatings are not discussed in the article.
U.S. Pat. No. 6,576,489 to Leung et al., issued Jun. 10, 2003 describes methods of forming microstructure devices. The methods include the use of vapor-phase alkylsilane-containing molecules to form a coating over a substrate surface. The alkylsilane-containing molecules are introduced into a reaction chamber containing the substrate by bubbling an anhydrous, inert gas through a liquid source of the alkylsilane-containing molecules, to transport the molecules in the vapor phase into the reaction chamber. The formation of the coating is carried out on a substrate surface at a temperature ranging between about 15° C. and 100° C., at a pressure in the reaction chamber which is said to be below atmospheric pressure, and yet sufficiently high for a suitable amount of alkylsilane-containing molecules to be present for expeditious formation of the coating. The liquid source of alkylsilane molecules may be heated to increase the vapor pressure of the alkylsilane-containing molecules.
While various methods useful in applying layers and coatings to semiconductor devices and MEMS have been discussed above and there is some description of the kinds of apparatus which may be employed to deposit the coatings, the apparatus description is minimal. The following references deal more with apparatus. U.S. Patent Application Publication No. US 2001/0028924 A1 of Arthur Sherman, published Oct. 11, 2001, pertains to a method of sequential chemical vapor deposition which is used to deposit layers of inorganic materials such as SiOx, Al2O3, TiO2, Si3N4, SiOxNy, and aluminum films doped with copper and silicon. U.S. Patent Application Publication No. US 2002/0076507 A1 of Chiang et al., published Jun. 20, 2002, describes an atomic layer deposition (ALD) process based on the sequential supply of at least two separate reactants into a process chamber. A first reactant reacts (becomes adsorbed) with the surface of the substrate via chemisorption. The first reactant gas is removed from the process chamber, and a second reactant gas reacts with the adsorbed reactant to form a monolayer of the desired film. The process is repeated to form a layer of a desired thickness. To reduce the process time, there is no separate purge gas used to purge the first reactant gas from the chamber prior to introducing the second gas, containing the second reactant. Instead, the purge gas also includes the second reactant. Several valving systems for gas flow to provide various mixtures of gases are described in detail.
The background information above provides a number of methods for generation of coatings which have considerable commercial applicability. The apparatus described for producing layers or coatings for use in electronic devices and/or micro-electromechanical systems devices enables application of the layers or coatings, but does not provide sufficient accuracy and repeatability in terms of the amount of the vaporous reactants provided to the substrate surface. As a result, the precise composition of the layer or coating which is desired may not be available. At other times, because of the improper ratio of various reactants relative to each other, or oversaturation by a precursor, reactants may polymerize and/or particulate agglomerations may be formed which act as surface contaminants. Further, the ability to reproduce the same coating reliably, time after time, is diminished due to lack of control over the precise amount of reactants supplied to the coating formation process. This decreases the product yield and affects the commercial viability of a coating process. It would be highly desirable to have a more accurate and reliable method of supplying precise quantities of the reactants to the process chamber and to the substrate surface for coating formation.